A sea otter eats a clam that it has taken from the shallow sandy bottom of Elkhorn Slough. Because sea otters have such a high metabolic rate, they eat up to 30% of their body weight each day in the form of clams, mussels, urchins, crabs and abalone. Sea otters are the only known tool-using marine mammal, using a stone or old shell to open the shells of their prey as they float on their backs. Elkhorn Slough National Estuarine Research Reserve, Moss Landing, California, USA